The present invention relates to an apparatus for converting radiation image information into electric signals, and particularly to an apparatus for converting low-dose X-ray image information into electric signals having a high resolution and/or a high S/N ratio (signal/noise ratio) The fundamental construction of an information reading apparatus for radiation image is disclosed in, for example, U.S. Pat. No. 3,859,527.
The technique of converting radiation image information into digital electric signals to obtain an image easy to use in diagnoses has recently become important. One example of the above-mentioned technique is disclosed in Japanese Patent Laid-Open No. 12,429/1980, and involves a process comprising irradiating a powder-screened data storage phosphor plate having a laser-stimulable phosphor powder BaFX Eu.sup.2+ (wherein X is Cl, Br, or I) applied thereto with X-rays and detecting image information accumulated in the powder-screened data storage phosphor plate in the form of scanning He-Ne laser-stimulated luminescence. However, scattering of laser beams for reading is notably large in the laser-stimulable phosphor powder-screened plate. Therefore, when the plate thickness is increased, the image is disadvantageously blurred. On the other hand, when the plate thickness is decreased, the X-ray absorption coefficient is lowered, leading to an increase in necessary X-ray dose, which is unfavorable in medical diagnoses.
There has been proposed a process comprising irradiating a plurality of highly powder-screened sheets piled up and comprising a plastic base and a laser-stimulable phosphor powder applied thereto with X-rays, reading the sheets one by one, and piling up the signals [see a report of Miyahara et al. titled "Digital Radiography (DR) System and Its Sensor," Hoshasen Zoh Kenkyu, Vol. 14, No. 1, pp. 7 to 15 (1984)]. Although the X-ray absorption coefficient can be improved in this process, there is a problem of the noise as well as the time in reading being doubled.
The technique of efficiently converting laser-stimulated luminescence into electric signals is necessary in detection of laser-stimulated luminescence by irradiation with laser beams as described above. As disclosed in Japanese Patent Publication No. 51,099/1985, there has conventionally been adopted a process comprising adjoining one edge of a transparent sheet to a scanning line on a data storage phosphor plate mentioned above while adjoining the other edge of the sheet to the light receiving surface of a photodetector to guide laser-stimulated luminescence to the photodetector. This prior art technique of converting laser-stimulated luminescence into electric signals involves the following problems.
A first problem is that laser-stimulated luminescence is taken in from the edge of the sheet since the system of the process comprises taking directional laser-stimulated luminescence into the transparent sheet and guiding the luminescence to the photodetector with repetition of total reflection. This is because most of the luminescence is released into the outside from the reverse surface of the sheet if luminescence is taken in from a flat surface of the sheet. Since the width of the edge of the sheet cannot be so large, an improvement is restricted in an efficiency of taking in laser-stimulated luminescence.
A second problem is that the edge of the transparent sheet must be brought as close as possible to the surface of the data storage phosphor plate in order to improve the efficiency of taking in laser-stimulated luminescence. Further, in order to read laser-stimulated luminescence by two-dimensional light scanning, the relative positions of the edge of the transparent sheet and the data storage phosphor plate must be varied with the progress of photoscanning. Thus, a transferring mechanism is needed. When the distance between the edge of the transparent sheet and the data storage phosphor plate is varied, the output signal of the photodetector is changed. Therefore, transfer by the transferring mechanism must be made while keeping the distance constant. However, this is not easy. The necessity for the transferring mechanism provided in the vicinity of the surface of the data storage phosphor plate presents a grave difficulty, for example, particularly in using a data storage phosphor plate while cooling the same.
A third problem is that a space for allowing laser beams to pass through must be provided in the proximity of the edge of the transparent sheet in order to directly irradiate the data storage phosphor plate with laser beams. This is a restraint to the improvement in the efficiency of taking in laser-stimulated luminescence.
A fourth problem is that laser beams diffusely reflected from the data storage phosphor plate is guided to the photodetector by way of the transparent sheet to lower the S/N ratio of output signals.
None of the above-mentioned problems can be fundamentally solved by the prior art technique. Further, it is necessary to always accurately control the positional relationship among the site of laser irradiation, the data storage phosphor plate, and the transparent sheet.
A fifth problem is that economically usable transparent sheets (e.g., made of an acrylic resin) generally has an absorption zone in the ultraviolet region, thus restricting the usable range of wavelengths of laser-stimulated luminescence.